Mixed potential gas sensors are currently being developed for combustion control and environmental monitoring applications. The devices typically are comprised of two different catalytic electrodes deposited on a solid electrolyte. Multiple reduction-oxidation reactions occurring between the gas phase and the electrodes cause mixed potentials of differing magnitude to develop at the dissimilar electrodes. The differences in heterogeneous kinetics, electrokinetics and the equilibrium potentials for these reactions all influence the device response to varying concentrations of analyte gas.
An example of a mixed potential device is a carbon monoxide sensor consisting of a porous platinum electrode and a porous gold electrode deposited on a zirconia based oxide electrolyte. The following electrochemical reactions occur on both the gold and platinum triple phase interfaces at differing rates:O2+4e−→2O2CO+O2→CO2+2e−
Mixed-potential sensors based on oxygen-ion conducting electrolytes have been studied since D. E. Willams et. al. demonstrated the working of a “Pt/YSZ/Au” CO-sensor operating at T≦400° C. Since that time several metal and metal-oxide electrodes have been used to design various mixed-potential sensors for the detection of CO, NOX and hydrocarbons. Although all these sensors do give a response in the presence of reducing-gases, their lack of stability, reproducibility and selectivity have hindered the commercial development of sensors based on this technology.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and, in part, will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.